It is generally known that the viscosity of a lubricant increases with a decrease of temperature, and the lubricant turns into a gel and is completely solidified with passage of time to lose fluidity at lower temperature than the crystallization point of wax contained in a lubricant.
The solidification of a lubricant at low temperature is considered to be attributed to crystallization of wax contained in the lubricant that is paraffin when cooling the lubricant and further to a network structure of the wax due to the cooling. In order to improve the fluidity of a lubricant, a typical method is to remove a wax component contributing to solidification to some extent by a dewaxing process at a certain low temperature, and furthermore to add a pour point depressant. The function of the pour point depressant is considered not to solubilize wax into lubricant, but to inhibit the wax from forming the network structure. In general, addition of a pour point depressant to a lubricant can lower the pour point. As a method for lowering pour points of lubricants by using polymer additives, Patent Document 1 discloses that polyalkyl(meth)acrylates with a limited average alkyl carbon number are used as pour point depressants for lubricants and viscosity index improvers.
By contrast, with social needs of fuel saving in recent years, there is a strong demand for lubricants having low viscosity at low temperature. As one means to meet the demands, high viscosity index base oils or a combination of high viscosity index base oils and common solvent refined base oils or synthetic lubricants have been used. However, it is known that, since the high viscosity index base oil and the solvent refined base oil are different in molecular structures, a pour point depressant having an effect when used for the solvent refined base oil does not always have an effect on the high viscosity index base oil. As pour point depressants specialized in lubricants using the high viscosity index base oils, combinations of particular polyalkyl(meth)acrylates have been developed in Patent Documents 2 and 3. However, the combination gives no satisfactory effect on the solvent refined base oils, so that the combination is not necessarily effective for lubricants in which the solvent refined base oils and high viscosity index base oils are blended at any given ratio. In view of the above, although pour point depressants for lubricants effective for both solvent refined base oils and high viscosity index base oils are required, pour point depressants for lubricants with quite satisfactory performance have not been obtained.    [Patent Document 1]
U.S. Pat. No. 2,655,479    [Patent Document 2]
Japanese Patent Application Publication No. JP-A-54-70305    [Patent Document 3]
Japanese Patent Application Publication No. JP-A-10-310758